1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for manufacturing the same, and more particularly to a liquid crystal display device in which pads of a liquid crystal display panel and/or wirings connected to the pads are protected from being corroded when the pads are connected to an external driver integrated circuit, and a method for manufacturing the liquid crystal display device.
2. Description of Related Art
In these days, electronic display devices are more widely used as information transmission media and various types of electronic display devices are used for industrial apparatus or home appliances. Such electronic display devices have been developed to have appropriate functions for various demands of the information era.
In general, electronic display devices display various information so that users can utilize such information. That is, the electronic display devices convert electric information signals outputted from electronic apparatus into light information signals recognized by the users through their eyes.
The types of electronic display devices include an emissive display device type and a non-emissive display device type. An emissive display device displays light information signals by using light emission, and a non-emissive display device displays light information signals by using reflection, scattering or interference of light. The emissive display device includes a cathode ray tube (CRT), a plasma display panel (PDP), a light emitting diode (LED) and an electroluminescent display (ELD). The emissive display device is called an active display device. Also, the non-emissive display device, called a passive display device, includes a liquid crystal display (LCD), an electrochemical display (ECD) and an electrophoretic image display (EPID).
The CRT has been used for a television or a monitor of a computer as a display device for a long time since it has a high quality and a low manufacturing cost. The CRT, however, has some disadvantages such as a heavy weight, a large volume and high power consumption.
Recently, the demand for a new electronic display device is greatly increased such as a flat panel display device having excellent characteristics of thin thickness, light weight, low driving voltage and low power consumption. Such flat panel display devices can be manufactured owing to the rapidly improved semiconductor technology.
Of the flat panel devices, the liquid crystal display (LCD) devices have been widely utilized for various electronic devices because an LCD device is thin and has low power consumption and high display quality almost same as that of a CRT. Also, the LCD devices can operate with a low driving voltage and can be easily manufactured so that the LCD devices are widely used for various electronic apparatuses.
The LCD devices are generally divided into a transmission type LCD device, a reflection type LCD device, and a reflection-transmission type LCD device. The transmission type LCD device displays information by using an external light source and the reflection type LCD device displays information by using natural light. The reflection-transmission type LCD device operates in a transmission mode for displaying an image using a built-in light source of the display device in a room or in a dark place where an external light source does not exist, and operates in a reflection mode for displaying the image by reflecting external incident light in the area having high illumination.
FIG. 1 is a schematic perspective view showing an LCD panel of a conventional liquid crystal display device, FIG. 2 is a plan view showing a conventional LCD panel and integrated circuits connected to the LCD panel for driving the LCD panel, and FIG. 3 is a cross-sectional view showing a thin film transistor and a pad region of the conventional LCD panel.
Referring to FIGS. 1 and 2, a liquid crystal device has an LCD panel 130 for displaying an image and driver integrated circuits 137 and 138 for generating image signals.
The LCD panel 130 includes a first substrate 100, a second substrate 102 located opposite to the first substrate 100, and liquid crystal 114 injected between the first substrate 100 and the second substrate 102.
A plurality of gate and data lines 104 and 106 are formed on the first substrate 100 in a matrix shape, and pixel electrodes 108 and thin film transistors (TFT) are formed at intersections of the gate lines 104 and the data lines 106. A color filter 112 and a transparent common electrode 110 are formed on the second substrate 102. The color filter 112 includes red·green·blue (R·G·B) pixels displaying predetermined colors while light passes through the pixels. Also, polarizing plates (not shown) are formed on the outsides of the first and the second substrates 100 and 102 for maintaining a direction of the light transmitting thereof according to an orientation of the liquid crystal 114.
Referring to FIG. 3, a thin film transistor 109 has a gate electrode 15 formed on the first substrate 100, a gate insulation layer 25 formed on the gate electrode 15 and the first substrate 100, an active pattern 30 formed on the gate insulation layer 25 where the gate electrode 15 is positioned, an ohmic contact layer pattern 35 formed on the active pattern 30, and source and drain electrodes 40 and 45 formed on the ohmic contact pattern 35. A passivation layer 50 is formed on the first substrate 100 on which the thin film transistor 109 is formed. The passivation layer 50 is comprised of an inorganic material or an organic material. A contact hole 80 is formed through the passivation layer 50 to expose the drain electrode 45. Also, contact holes (not shown) are formed through the passivation layer 50 to expose gate terminals and drain terminals in a pad region.
The gate electrode 15 is connected to the gate line 104, and the source electrode 40 is connected to the data line 106. The drain electrode 45 is connected to the pixel electrode 108. Thus, when a scanning voltage is applied to the gate electrode 15 through the gate line 104, a signal voltage passing the data line 106 is applied from the source electrode 40 to the drain electrode 45 through the active pattern 30. When the signal voltage is applied to the drain electrode 45, a potential difference is generated between a common electrode 110 of the second substrate 102 and the pixel electrode 108 connected to the drain electrode 45. Then, the molecular arrangement of the liquid crystal 114 injected between the pixel electrode 108 and the common electrode 110 is changed, so light transmissivity of the liquid crystal 114 is varied. Thus, the thin film transistor 109 serves as a switching device for operating pixels of the LCD panel 130.
In addition, first pads 133 and second pads 134 are formed on the LCD panel 130 as shown in FIG. 2. The first pads 133 are prolonged from the gate line 104 and the second pads 134 are prolonged from the data line 106. The first pads 133 are connected to a first integrated circuit 137 generating the scanning voltage, and the second pads 134 are connected to a second integrated circuit 138 generating the signal voltage. Hence, the scanning voltage generated from the first integrated circuit 137 is applied to the gate line 104 through the first pads 133, and the signal voltage generated from the second integrated circuit 138 is applied to the data line 106 through the second pads 134. Various methods can be utilized for connecting the first pads 133 to the first integrated circuit 137 or connecting the second pads 134 to the second integrated circuit 138. Typically, bumps for having connection with the pads are formed on electrodes of the integrated circuit, and then the pads are connected to the integrated circuit using the bumps.
In general, a tape automated bonding (TAB) method is used for connecting the pads to the integrated circuit. According to the TAB method, a lead of a TAB package attached to an electrode of the integrated circuit is attached to the LCD panel after a film having metal lines attached thereto and the electrode are connected to each other using bumps. That is, after mounting the integrated circuit on the outside of the LCD panel, the electrode of the integrated circuit and the electrode of the LCD panel are electrically connected with each other using the film to which metal lines are attached.
Also, a chip on glass (COG) method can be utilized for connecting a driver integrated circuit to an LCD panel instead of the TAB method. According to the COG method where the driver integrated circuit is directly installed on the LCD panel, the integrated circuit is attached to the substrate of the LCD panel only using bumps and an anisotropic conductive film (ACF) without the film used in the TAB method.
FIG. 4 is a cross-sectional view illustrating a conventional COG method for connecting an integrated circuit to a liquid crystal display device.
Referring to FIG. 4, an ACF resin 153 is loaded on a substrate 180 corresponding to pads 181 of an LCD panel. Then, an integrated circuit 140 on which bumps 144 are formed is attached to the substrate 180 via a thermal compression method. As a result, conductive balls 154 dispersed in the ACF resin 153 are pressed by the bumps 144 and the pads 181 so that an insulating layer (not shown) enclosing the conductive balls 154 are broken. Thus, an electrode of the integrated circuit 140 and the pads 181 of the LCD panel are electrically connected. Subsequently, the integrated circuit 140 is heated to harden the ACF resin 153 which has been softened due to the compression process, thereby attaching the integrated circuit 140 to the pads 181 of the substrate 180.
The COG method is widely used for small and midsize panels to enhance durability of a mobile product which is subject to external impact or vibration, because it is simply performed in comparison with the TAB method and the area ratio of an LCD panel increases in a liquid crystal display device.
However, when pollutants such as moisture or chemicals penetrate into exposed portions of the pads connected to the bumps of the integrated circuit, the pads and the wirings connected thereto may be corroded due to an electrochemical reaction between the pollutants and the pads and/or the wirings. Thus, the metal corrosion occurs and the electrical signal of the wirings is interrupted to thereby cause a failure in driving the liquid crystal display device.
Such metal corrosion may also occur due to electrolysis caused by the potential difference between the adjacent pads. That is, when the potential difference is generated between two metals, electrons of a positive (+) metal migrate into a negative (−) metal so that the positive metal is lack of electrons and thus, is corroded finally. Particularly, as the potential difference between the adjacent pads is larger, the pads and/or the wirings connected thereto are corroded easily.
For example, a gate signal applied to a gate driver integrated circuit of the liquid crystal display device may include voltage Von, voltage Voff, a vertical clock signal and a vertical start pulse signal. Voltage Von and voltage Voff are +15V and −7V, respectively, and thus, the potential difference therebetween becomes 22V. Such high potential difference increases the electron migration to thereby cause corrosion in the pad to which voltage Von is applied, which results in the driving failure of the gate driver integrated circuit.